Suppliers of energy feedback devices for frequency converters remind you that currently, simple energy consumption braking is widely used in AC frequency conversion speed control systems, which have disadvantages such as wasting electrical energy, severe resistance heating, and poor fast braking performance. When asynchronous motors frequently brake, using feedback braking is a very effective energy-saving method and avoids damage to the environment and equipment during braking. Satisfactory results have been achieved in industries such as electric locomotives and oil extraction. With the continuous emergence of new power electronic devices, increasing cost-effectiveness, and people's awareness of energy conservation and consumption reduction, there is a wide range of application prospects.
Feedback braking principle
In the variable frequency speed regulation system, the deceleration and stopping of the motor are achieved by gradually reducing the frequency. At the moment when the frequency decreases, the synchronous speed of the motor decreases accordingly. However, due to mechanical inertia, the rotor speed of the motor remains unchanged, and its speed change has a certain time lag. At this time, the actual speed will be greater than the given speed, resulting in a situation where the back electromotive force e of the motor is higher than the DC terminal voltage u of the frequency converter, that is, e>u. At this point, the electric motor becomes a generator, which not only does not require power supply from the grid, but can also send electricity to the grid. This not only has a good braking effect, but also converts kinetic energy into electrical energy, which can be sent to the grid to recover energy, killing two birds with one stone. Of course, there must be an energy feedback device unit for automatic control in order to achieve it. In addition, the energy feedback circuit should also include AC and DC reactors, resistance capacitance absorbers, electronic switches, etc.
As is well known, the bridge rectifier circuit of general frequency converters is three-phase uncontrollable, so it cannot achieve bidirectional energy transfer between the DC circuit and the power supply. The most effective way to solve this problem is to use active inverter technology, and the rectifier part adopts reversible rectifier, also known as grid side converter. By controlling the grid side inverter, the regenerated electrical energy is inverted into AC power with the same frequency, phase, and frequency as the grid, and fed back to the grid to achieve braking. Previously, active inverter units mainly used thyristor circuits, which can only safely perform feedback operation under stable grid voltage that is not prone to faults (grid voltage fluctuations not exceeding 10%). This type of circuit can only safely perform feedback operation of the inverter under stable grid voltage that is not prone to faults (with grid voltage fluctuations not exceeding 10%). Because during power generation braking operation, if the grid voltage braking time is greater than 2ms, commutation failure may occur and components may be damaged. In addition, during deep control, this method has low power factor, high harmonic content, and overlapping commutation, which will cause distortion of the power grid voltage waveform. Simultaneously controlling complexity and high cost. With the practical application of fully controlled devices, people have developed chopper controlled reversible converters using PWM control. In this way, the structure of the grid side inverter is completely the same as that of the inverter, both using PWM control.
From the above analysis, it can be seen that to truly achieve energy feedback braking of the inverter, the key is to control the grid side inverter. The following text focuses on the control algorithm of the grid side inverter using fully controlled devices and PWM control method.
Feedback braking characteristics
Strictly speaking, the grid side inverter cannot be simply referred to as a "rectifier" because it can function as both a rectifier and an inverter. Due to the use of self turn off devices, the magnitude and phase of the AC current can be controlled through appropriate PWM mode, making the input current approach a sine wave and ensuring that the power factor of the system always approaches 1. When the regenerative power returned from the inverter by the motor deceleration braking increases the DC voltage, the phase of the AC input current can be reversed from the phase of the power supply voltage to achieve regenerative operation, and the regenerative power can be fed back to the AC power grid, while the system can still maintain the DC voltage at the given value. In this case, the grid side inverter operates in an active inverter state. This makes it easy to achieve bidirectional power flow and has a fast dynamic response speed. At the same time, this topology structure enables the system to fully control the exchange of reactive and active power between the AC and DC sides, with an efficiency of up to 97% and significant economic benefits. The heat loss is 1% of the energy consumption braking, and it does not pollute the power grid. The power factor is about 1, which is environmentally friendly. Therefore, feedback braking can be widely used for energy-saving operation in energy feedback braking scenarios of PWM AC transmission, especially in situations where frequent braking is required. The power of the electric motor is also high, and the energy-saving effect is significant. Depending on the operating conditions, the average energy-saving effect is about 20%.